Centrifugal clutches are widely used to transmit torque from an engine to a user device. Generally these are used for devices having small, or at the most moderate, horsepower ratings. Their use in small vehicles such as scooters and golf carts are well known. The clutch art is well-developed for such devices. The transmissions are generally sizable, and weight and bulk are of little concern, and in these fields some increases in cost of components are tolerable.
In recent years the use of fractional horsepower 2 cycle internal combustion engines has become commonplace. Their small engines drive relatively small devices. Brush cutters and flexible string trimmers are well-known examples. These are hand-carried appliances. A lighter-weight device which will accomplish the same results as a heavier device is likelier to sell.
Furthermore, the sales price of these devices is surprisingly low, and the market is very competitive as to price. It follows that any component of the device which can perform the same function as a more expensive component, can result in important cost advantages. It should be remembered that the sales price of the appliance is the resultant of many factors, of which the manufacturing cost is an important one. Any increase in cost at that level is magnified at least several times in the retail price of the device, because that is how commercial pricing is done. Conversely, the saving of even a few cents at the manufacturing level can give the sales department an important edge, and in head-on price competition, can generate substantial profits.
Cost and pricing considerations aside, there still remains the objective of producing a reliable and long-lived appliance. It does no good to reduce manufacturing costs merely to lose the advantage by way of increased warranty repair work.
A centrifugal clutch has a driver portion and a driven portion. The driver portion is connected directly or through some transmission means such as a solid shaft or a flexible cable, to an engine. It includes throw weights which at a sufficient rotational velocity, move radially outward to engage the driven portion and drive it.
The driven portion includes a circular drum assembly which the throw weights engage. This driven portion has an output shaft to which power transmission means is coupled. The output shaft is journaled to a housing by a bearing. The coupling is generally made to a rather small-sectioned power transmission element such as a square-shaped end fitting on a flexible cable drive or a square shaped end on a solid shaft.
The rotational velocities of these drives is surprisingly large, and there are considerable peak-load torque forces. True concentricity of the drum and the output shaft is necessary, and so is a very strong, wear-resisting coupling between the output shaft and the driven device such as the input shaft of a brush cutter blade.
These requirements, and the desirability of a lower cost item, are in known devices at cross-purposes with one another. The concentricity of the drum and the output shaft may readily be lost by heat treating an assembled structure that is heat-treated in order to provide sufficient strength and surface hardness. Also, in order to provide sufficient bearing strength for the joinder in the output shaft, a relatively expensive heat-treatable alloy must be used. As a consequence, to make the drum assembly of drum and output shaft, great care must be taken in assembly and heat treating, and relatively costly materials must be used for both.
It is an object of this invention to provide a drum assembly for a centrifugal clutch whose parts can be assembled by expedient brazing operations, and which can be made mostly of relatively inexpensive materials Which do not require heat treatment, and to provide an exceptionally inexpensive part where hardness is needed.